The present invention relates to a digital audio broadcast receiver, more particularly to a method used by the receiver to find a receivable signal broadcast in compliance with Recommendation BS.774 of the Radiocommunication Sector of the International Telecommunications Union (ITU-R), entitled "Service requirements for digital sound broadcasting to vehicular, portable and fixed receivers using terrestrial transmitters in the VHF/UHF bands."
The service requirements stated in this Recommendation include sound quality comparable to that of a compact disc recording, improved performance under multipath reception conditions, and the capability to transmit not only audio programming but also program-related data and value-added services to an inexpensive, mass-producible receiver. These requirements have been met by the Eureka Digital Audio Broadcasting (DAB) system, embodied in European Telecommunications Standard ETS 300 401. The Eureka DAB system has been tested extensively in Europe and elsewhere, and has been recommended by the ITU-R for use internationally.
Digital audio broadcasting has many advantages over analog broadcasting, especially for mobile receivers, but conventional digital audio broadcast receivers have also had the disadvantage of sluggish tuning, as compared with analog receivers. When the user of a conventional digital audio broadcast receiver selects a known broadcast frequency and program, before reception can begin, the receiver must synchronize itself with the broadcast signal and accurately tune itself to the broadcast frequency. In the Eureka DAB system, this is done by detecting a pair of synchronization symbols that are broadcast at intervals of ninety-six milliseconds (in one transmission mode) or twenty-four milliseconds (in another transmission mode). Each of the synchronization symbols must be detected repeatedly before synchronization and fine tuning are accomplished, so the process takes a certain amount of time.
Even after synchronization and tuning have been acquired, reception cannot begin immediately. Since several programs may be multiplexed in the same broadcast signal, the receiver must first receive enough program-related data to find the selected program. Next, the receiver must begin de-interleaving and decoding the audio data. In the Eureka DAB system, the de-interleaving delay alone is at least four-tenths of a second. Altogether, a quite noticeable interval of time elapses from the pressing of the selection button until the desired program is actually heard.
This delay may be disconcerting to users who are accustomed to instant reception from analog receivers. Before the receiver is able to begin reproducing the broadcast the user has selected, the user may conclude that the selected broadcast is not on the air, and switch to another selection.
If a digital audio broadcast frequency has not been preset in the receiver, the receiver must scan through a frequency band in which digital audio broadcast signals might appear, testing frequencies one by one for the presence of a receivable broadcast signal, and undertaking the process described above each time a receivable signal is found. To the user, who cannot immediately tell when a broadcast signal has been found, but must wait for the fine-tuning and other steps to be completed, the scanning process may seem aggravatingly slow.
If the scanned frequency band contained only digital audio broadcast signals, this last problem could be partially solved by a received signal strength indicator that indicated when a carrier signal had been picked up. Digital audio broadcast frequencies, however, are often assigned in a band already populated by analog broadcast signals. If the receiver simply indicates the presence of a carrier signal, the user cannot immediately tell whether the signal is a receivable digital audio broadcast signal, or a non-receivable signal of some other type.
A receiver may have both analog and digital receiving circuits, and be capable of receiving both types of broadcasts. Even in this case, the receiver requires a way of speedily discriminating between analog and digital broadcast signals, so that appropriate action can be taken.